1. Field of the Invention
The present invention relates to a system for the downhole separation of fluids and, more particularly, to such a system that separates oil from water within a wellbore and that disposes of the separated water within the wellbore.
2. Description of Related Art
In many oil fields around the world the percentage of water recovered with oil from subterranean wellbore has risen to be greater than the percentage of oil. In fact, in many fields, the percentage of oil has decreased to about 20% in an excellent field to less than 2% in a relatively poor field. Therefore, the operator must lift to the surface, and then dispose of, the resulting tremendous volume of water. This situation wastes energy because of the power needed to operate pumps to lift and separate the water, and causes an environmental problem. In many locations the separated water cannot be disposed of on the surface, so the water must be transported to a remote well site to be reinjected into a subterranean formation. There is a need for a method and related system for separating the oil from water downhole so that the quantity of water recovered to the earth's surface can be minimized, and hopefully, eliminated.
One method of downhole oil and water separation is disclosed in U.S. Pat. Nos. 5,296,153 and 5,456,837, wherein wellbore fluids are drawn through a hydrocyclone that separates the oil from water. The separated water is then introduced into a first pump to force the water into a subterranean formation, which is isolated from the formation from which the oil and water mixture is recovered. The separated oil is introduced into a second pump to force the oil to the earth's surface for processing.
In field trials of the fluid separation system disclosed in U.S. Pat. No. 5,296,153, it became apparent that a means for controlling one or more aspects of the downhole separation system is desired. In the past, backpressure control on the downhole hydrocyclones has been provided by a valve on the underflow of the hydrocyclones, as is disclosed in U.S. Pat. Nos. 4,770,243, 4,805,697, 4,900,445, 5,093,006, and 5,456,837. Various inlet flow controls are disclosed in U.S. Pat. Nos. 4,976,872 and 4,983,283. Further, recycle of separated water has been disclosed in U.S. Pat. No. 4,900,445.
One reason for the need for this control is that hydrocyclones are incapable of separating fluid components over the full range of component ratios. This is primarily due to the need for specific geometric configurations, port sizings of the inlets and outlets, and flow rates for specific fluid mixtures. For example, "deoiling" hydrocyclones are used when the fluid mixture is water-external, i.e., a water medium surrounding oil droplets. This means that the deoiling hydrocyclones are designed to separate relatively low viscosity mixtures, with a relatively small portion of the inlet flow exiting the overflow of this hydrocyclone. On the other hand, "dewatering" or "dehydrating" hydrocyclones are used when the fluid mixture is oil-external, i.e., an oil medium surrounding water droplets. This means that the dewatering hydrocyclones are designed to separate higher viscosity mixtures, with a relatively large portion of flow exiting the overflow of this hydrocyclone. Usually, oil is more viscous than water, and this has little effect on the viscosity of a water-external emulsion, since the viscosity of the water dominates. However, an oil-external emulsion will have a viscosity closer to that of the oil.
As a generalization, oil/water fluid mixtures with more than about 65% water are water-external, and a deoiling hydrocyclone would be selected to remove the oil. Likewise, mixtures with less than about 35% water are oil-external, and a dewatering hydrocyclone would be selected to remove the water. The exact percentage of water where the mixture will change from being water-external to oil-external varies with the oil's properties and wellbore conditions, such as temperature and pressure. The basic problem to be overcome is that deoiling hydrocyclones cannot operate effectively on oil-external mixtures, and dewatering hydrocyclones cannot operate effectively on water-external mixtures.
Therefore, this problem leads to the operational problem of having to select the proper hydrocyclone to use, for example, for a 50% water mixture where neither configuration of hydrocyclone can effectively operate. Further, fluid mixtures recovered from subterranean reservoirs change over time due to the depletion of the oil, reduction of formation pressure, and the like, so that in the past the well operator would need to remove and exchange the hydrocyclones as the fluid mixtures changes.
There is a need for a fluid separation system that can be used with one or more configurations of fluid separators, and can be adjusted to ensure that acceptable separation efficiency can be achieved over a broad range of fluid component ratios for a given fluid separator, as well as eliminating the need to replace the existing fluid separators when the fluid mixtures change.